1. Field of the Invention
The present invention relates to digital control variable delay circuits and more particularly to a digital control variable delay circuit for receiving n pairs of phase-shifted clock signals as input signals and varying the amplitudes of the input signals in response to digital control signals supplied separately to generate signals each having the varied amplitude as output signals.
2. Description of the Related Art
A voltage-controlled phase shifter having an unlimited range is included in latest techniques which can be applied to variable delay circuits. For example, PCT Japanese Translation Patent Publication No. 9-512965 discloses such a voltage-controlled phase shifter. The voltage-controlled phase shifter selects two adjacent signals from among a plurality of multiphase signals and outputs phase-shifted signals by carrying out weighting and adding to the selected signals in accordance with an analog control voltage by the use of an interpolator.
Techniques related to the other variable delay circuits are disclosed in, for example, Japanese Unexamined Patent Publication Nos. 63-31214 and 6-152343. In addition Japanese Unexamined Patent Publication No. 9-214334 discloses a digital delay interpolator. Further, Japanese Unexamined Patent Publication No. 11-284497 discloses a programmable delay generator.
The above-mentioned voltage-controlled phase shifter determines the amount of phase shift in accordance with an analog control voltage. Therefore, the voltage-controlled phase shifter is susceptible to external noise. For example, in an environment where an external factor such as power source noise considerably affects on the shifter, the analog control voltage varies due to the noise, resulting in a fluctuation of the phase of a phase-shifted output signal. In addition, it is necessary to control a phase selector for selecting two signals and a phase interpolator for weighting and interpolating the two signals. Consequently, the above-mentioned voltage-controlled phase shifter has complicated circuitry. In addition, disadvantageously, the relation between the analog control voltage and the amount of phase shift is not uniquely determined due to a manufacturing variation in circuit elements or fluctuations in supply voltage and ambient temperature.
For this reasons, the above-mentioned voltage-controlled phase shifter is hard to apply to a variable delay circuit under present circumstances.